1. Field of the Invention
The invention relates to an improved process for producing a catalyst useful for the epoxidation of ethylene to ethylene oxide. It relates to a process wherein a solid support is impregnated with a silver-containing compound and one or more optional promoters, and is calcined to convert the impregnated support into a catalyst. The improvement of this process is that a two-stage calcination is conducted. That is, the impregnated support is subjected to a first calcination in an air atmosphere, and a second calcination in an inert gas atmosphere.
2. Description of the Related Art
In the catalytic epoxidation of ethylene, modern silver-based supported catalysts are highly selective towards ethylene oxide production. Preparation of such catalysts typically involves impregnating a solid support with a silver solution and optional promoters such as transition metals or alkali metals, and thereafter calcining the impregnated support in order to reduce the silver solution to metallic silver, and to separate volatiles from the catalyst. U.S. Pat. No. 4,916,243 shows silver catalysts for ethylene oxidation to ethylene oxide prepared by impregnating an inert support with a silver/amine and silver lactate solutions. The impregnated carriers are than heat treated on steel belt transported through a heating zone for 2.5 minutes, the heating zone being maintained at 500° C. by passing hot air upward through the belt, or at 400° C. for 4 minutes.
It is further known in the art to conduct the calcination/heating of such impregnated supports in a nitrogen atmosphere. For example, U.S. Pat. No. 5,444,034 relates to silver catalyst preparation wherein a support is impregnated with a hydrocarbon solution of a silver salt of an organic acid and activated in stages up to a temperature of 500° C. under an inert gas such as nitrogen.
While several conventional processes conduct their calcinations in inert gas atmospheres such as nitrogen, these inert gas calcinations are expensive. Attempts have thus been made to calcine an impregnated support in an air atmosphere, to reduce costs. However, several disadvantages result from calcination in air alone. First, it has been observed that air calcination of an impregnated support produces a gray-black material that is less uniform in color than a catalyst which is produced by calcination in an inert gas such as nitrogen. Air calcination is typically conducted at lower temperatures to ensure that the catalyst is not damaged in the production process. This lower temperature calcination undesirably leaves greater amounts of residual organic species on the surface of the produced catalyst than during a higher temperature nitrogen calcination. Thus it takes longer to remove the residual organic species resulting from an air calcination process than from a nitrogen calcination during catalyst start up. This represents lost production time as well as reduction in the profitablility of the process.
It has now been unexpectedly found that it is beneficial to conduct two calcinations of an impregnated support. According to this invention, a first calcination is conducted in an air atmosphere, and a second calcination is conducted in an inert gas atmosphere, such as a nitrogen atmosphere. This two-stage calcination produces an improved catalyst which takes less time to manufacture, and contains fewer organics left over under standard conditions of air calcination alone, while costing less than calcination in an inert gas alone.